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The most common reason to run `mach clang-format` is that you have made some changes and they need to be reformatted. So why not detect changes and run clang-format on those changes?
You can always run `./mach clang-format .` to force it to check everything according to its rules, but that's not likely to be needed.
I also ran autopep8 on `mach` and it fixed a few things. I wonder if we should insist on that for python files...